Optical coherence tomography (“OCT”) techniques provide exemplary cross-sectional images of biological samples with resolution on the scale of several to tens of microns. Contrast in conventional OCT results from differences in the optical scattering properties of various tissues and permits imaging of tissue microstructure. It has been demonstrated that Fourier-Domain OCT (“FD-OCT”) provides significantly improved sensitivity, enabling high-speed imaging. FD-OCT has been implemented in two configurations, spectral-domain OCT (“SD-OCT”) and optical frequency domain imaging (“OFDI”). In exemplary SD-OCT arrangement as shown in FIG. 1(a), a spectrometer can be used to record spectral fringes that result from the interference of a reference beam with light reflected from a sample. In the exemplary OFDI arrangement as shown in FIG. 1(b), a narrowband wavelength-swept source and a single detector are used to record the same interferogram.
The exemplary OFDI arrangements and methods, however, may become the preferred imaging modalities for several applications since they may be less prone to motion artifacts associated with endoscopy and can provide a larger depth range. However, the maximum ranging depth can typically be limited by the instantaneous line-width (coherence length) of the laser source. For a number of OFDI sources, there may be a tradeoff between instantaneous line-width, tuning speed, output power, and tuning range, which ultimately limits the useful ranging depth. Several methods have been described to avoid the ambiguity between positive and negative depths and increase ranging depth by measuring quadrature interference signals or using both sides of the coherence range. Continued development of wavelength-swept laser sources can provide further improvements in imaging speed and resolution. These advantages are important in several exemplary OCT applications, including Barrett's esophagus screening and coronary imaging. As such, imaging in many applications may require increasing ranging depth.
One of the objects of the present invention is to overcome the above-described deficiencies.